From what I've heard, a bullet traveling at supersonic speed will make sonic cracks at regular intervals. A crack at 50 yards, a crack at 100 yards, or something like that. I've heard that in many cases the bullet will not create a sonic boom until it's traveled some distance from the shooter.

Is this correct, and if so, do the intervals change depending on the speed of the bullet, air pressure and other factors? If you can adjust the interval by changing the load of the ammunition, it seems that it would be possible to use a suppressor and create the illusion that shots are coming from closer to the target than they actually are.

Yithian

January 25, 2008, 04:47 PM

They don't "crack" at intervals.
They "crack" the entire distance traveled, until they fall beneath the sound barrier.

The sonic boom follows behind the bullet.
Your "intervals" may be just echos of it; off fauna, structures and hills.

Jim Watson

January 25, 2008, 04:58 PM

The shockwave can reflect off objects near its path.
General Hatcher said a Springfield .30-06 with Maxim Silencer was fired on a range with a row of telegraph poles down one side. He said the shockwave echoes sounded like a machine gun burst. But that is the only sort of "interval" you would get. Otherwise, you hear the crack shortly after the bullet goes by.

thallub

January 25, 2008, 05:14 PM

Browning experimented with silencers for machine guns in the early 1900s. Someone who worked with him said that there would be a loud crack that would appear to come from every stationary that the bullet passed. There are some early photos of Browning shooting his suppressed Maxim machine gun along a line of telephone poles.

Snipers take advantage of this crack. It may seem like the bullet came from nearby trees that the bullet passed, even though the round was fired from 500 yards away.

During WWII the OSS used some genuine sub-sonic silenced rounds in caliber .30 M1. The case was made in two pieces from brass bar stock. The base threaded onto the body of the case. There was a very small powder charge that pushed a captured sealed piston into the bullet.

There were also silenced rounds in .45 caliber. During Viet Nam the US Army developed a silent round for the M79 grenade launcher. The gas was contained in a flexible bellows.

MTMilitiaman

January 25, 2008, 05:36 PM

A gunshot is comprised of two separate auditory events. First, if the bullet is supersonic, there will be a sharp "crack" as the bullet passes the observer. This sound is omni-directional--that is, it doesn't give a clear indication of where the shot was fired from. The sound always comes from the direction of the bullet as it passes the observer. If the shot came from in front of you and is aimed passed your position, and the bullet crosses you on your left, the snap or crack of the bullet will come from your left.

For example, when we pulled targets in the pit at boot camp, we quickly became accustomed to the snapping sound of the bullets passing over our heads.

The second part of the gunshot is the sound of the high pressure gases make when they exit the muzzle. This is heard as a loud BOOM that most relate to as a gunshot. It travels at the speed of sound, which IIRC, is about 1130 fps at sea level. And it is unidirectional in that it does tell the observer the direction it came from.

When I first started hunting, my dad decided to show me something. He had me hide behind a 40" Ponderosa Pine located about 6 feet to the side of the 200 yard target on our private range on my grandpa's property. Then he shot a couple rounds from the bench 200 yards away. The distinctive crack, followed a split second later by the characteristic BOOM were clearly separated. He did this so I knew what it sounded like to have a bullet fired in my direction, so if I was ever out hunting I would know what incoming fire sounded like. (His advice was that if I ever heard that sound I was to hit the ground, get behind something, and start screaming obscenities at the shooter's mother, and if that didn't work, return fire.)

Because the BOOM travels at the speed of sound while the bullet often surpasses the speed of sound, the BOOM follows the crack at increasing intervals the farther down range you get. A typical high powered rifle bullet can reach 1000 yards in 1.5 seconds or less pretty easily. Sound takes a little over two and a half seconds to travel the same distance, so if you were being shot at from 1000 yards away, you'd hear a crack as the bullet passed you then a BOOM from the direction of the gunshot a little over a second later.

Sound suppressors capture the escaping gases and allow them to cool and dissipate over a longer period of time. This greatly reduces the BOOM, even if supersonic ammunition is used. This means that if someone shoots at you with a suppressed high powered rifle, all you hear is the crack, which gives no indication of where the shot came from. It tells you which direction the bullet was relative to you when it crossed your position, but not which direction it was traveling or where it originated from.

Using sub-sonic ammunition, reduces even this.

carguychris

January 26, 2008, 09:06 AM

Wow, great post, MTMilitiaman! :cool:

I'll go a little bit further. The crack is caused by a shockwave that lags behind the bullet. Observers will hear the crack as the shockwave passes them, so as the bullet travels further and further downrange, observers will hear an increasingly larger time interval between the crack and the BOOM as the sonic shockwave gradually "outruns" the sound of the gun being fired.

This effect often causes confusion amongst untrained witnesses to crimes where several shots are fired. This is why police often get differing accounts of the number of shots, often depending on the position of the witness relative to the shooter. People close to the shooter may hear no discernable difference between the shockwave and the gun's report, while people further away will often hear more "shots" because of the sonic shockwaves, and may become further disoriented by echoes.

Possibly the best real-world example is 44 years of debate about how many shots were fired during the Kennedy assassination*, where there were scores of witnesses located at varying distances from the shooting in a canyon of buildings closed on 3 sides but open on the 4th, causing echoes galore.

*I don't want to start an extensive discussion about various Kennedy conspiracy theories here. I'm just citing this as an example!

DesertDawg

January 26, 2008, 09:43 AM

The sonic BOOM is caused by the nose of the mass of whatever is travelling past you at a super-sonic rate of speed (I think that the sound barrier is 700+ MPH at sea level).

In the case of the space shuttle, even though it appears to be only ONE massive air/space craft, the nose of it is one mass, and the tail is another. I've been at shuttle landings, and there are always TWO distinctive BOOMS that can be heard in rapid succession. The BOOM is continuous as long as the mass is travelling at super-sonic speed, but the SOUND of the BOOM is not heard continuously, since the shock wave is directed to your ears only at a certain angle. My dad (now deceased) was an aircraft engineer, and claimed that the BOOM would be continuous if you were flying in another aircraft at the same speed, and at a certain angle from the other aircraft.

W.E.G.

January 26, 2008, 08:21 PM

This whole "crack" thing, in combination with echoes, is certainly why people reported hearing gunshots from so many different locations during the Kennedy assasination.

All reflected sounds aside, the "crack" of a bullet will appear to originate from the point at which the bullet passes closest to the listener.

So, if you are standing in an open field, without sound-reflective surfaces, as you are facing a shooter 300 yards away, and the shooter fires a high-velocity bullet past you several feet to your left you will hear a very loud "crack" that will appear to originate from exactly 90 degrees to your left. You may hear some bit of the report of the gun, but the report of the gun will be quite indistinct compared to the "crack" on your left. In the open, the report of a rife dissipates and diminishes in a surprisingly short distance.

If you are the target, at any distance, the shooter will be located roughly ninety degrees "off" from the apparent point of origin of the loudest sound of the projectile, unless the sound is affected by sound-reflective surfaces. If the "crack" is reflected, it will be more effective to try to hear the report of the weapon(s).

Suffice to say, this analysis will be difficult to perform on the fly if you are the target.

Lastly, the report (combined with the "crack" at the muzzle) of a suppressed .223 sounds about like an un-suppressed .22 magnum. Don't plan on shooting it much without ear protection. But, you could probably shoot it once or twice indoors if you had to without severe, permanent hearing damage.